@Opus 17
<<the rules for 2023 are different than the rule for the future.>>
Can I assume you mean that the rules that this couple need to follow beginning in 2024 are a different set of rules than the rules they followed in 2023 (which I concur with)?
but the IRS rules themselves for 2023 and 2024 are (so far) the same - they are not different.
if the current living arrangement continues and marital status continues into 2024, Dad would be the custodial parent and would file HOH, claiming all three children. Mom would file MFS as she had no qualified children. Dad would sign form 8332 to permit Mom to claim CTC for two of the children (or however many children Dad permitted).
